The water-steam cycle of a steam power plant or a combined-cycle power plant substantially comprises a boiler or heat recovery steam generator as the steam-generating means, a steam turbine, a condenser, a feedwater vessel/degassing means, as well as pumps and pipes. In this system, there are very high demands imposed on the purity of the working medium, and a special water-chemical procedure is used to lower the levels of corrosion and deposits.
By way of example, to make optimum use of the heat of the exhaust gas from the gas turbine, the heat recovery steam generator of a combined-cycle installation comprises up to three pressure stages for generating steam at different pressure levels. A pressure stage includes an economizer or preheater and an evaporator, and also if appropriate a steam drum and a superheater.
To ensure correct chemical operation of the water-steam cycle, the working medium has to be degassed, which can be carried out, for example, by means of a thermal process. Degassing of the working medium is particularly important if the system was at ambient pressure or had been opened, i.e. in particular after inspection and/or maintenance work or during cold starts. The degassing is also important if large quantities of make-up water are flowing in.
According to the prior art, the thermal degassing takes place either in the condenser, in the feedwater tank/degassing means, or by means of a degassing means which has been fitted to the low-pressure drum. In particular the latter two variants require high investment costs on account of the additional components required. An additional factor in these variants is that they are disadvantageous in terms of energy on account of the need for heating steam in order to heat a liquid which has been slightly cooled by 5–20 K to saturation temperature. The higher the pressure level of the degassing means, the greater this drawback becomes.
With regard to the thermal degassing of the working medium, it should be ensured that the use of heating heat for the degassing procedure takes place in accordance with the prevailing qualitative and quantitative requirements, in order to ensure that the overall efficiency of the installation is reduced as little as possible and only for a limited time.
In this context, reference is made to EP 0 359 735 B1, which proposes a heat recovery steam generator in which the steam drum is provided with an integrated thermal degassing means, and which steam drum, in addition to the function of separating the water-steam mixture, also has the function of storing the feedwater. In other words, the abovementioned document substantially involves avoiding the need for the feedwater tank/degassing means in the water/steam cycle by suitable modification of the steam drum. However, the patent fails to give any details as to how this integrated thermal degassing is configured and as to what means are made available for the degassing. The figure merely shows that the means could be a fitted trickling degassing means.
If the systems are functioning perfectly and if the installations operating in the subatmospheric pressure range are properly sealed, it is not absolutely inevitable that degassing will be required throughout the entire operating time of the installation. For this reason, it is also not sensible to ensure permanent high-quality degassing with high investment costs and a constant power loss. Rather, it is more appropriate to use additional measures to ensure appropriate degassing in the event of special conditions or when time-limited procedures are being implemented, but to operate the installation without this additional degassing during normal operation.